Green Technology for Reducing Postharvest Losses and Improving the Nutritional Quality of Fresh Horticultural Produce

Abstract

Fresh fruits and vegetables provide essential nutrition to the diet, and it is critical to maintain product quality and nutrition from harvest through to the consumer. Fresh fruit and vegetables are still ‘alive’ even after detached from the plants and continue to respire. Besides, the climacteric fruits ripen after harvest. Therefore, it is important to manage the ripening process and prevent decay to reduce postharvest losses. In addition, foodborne illnesses are a major public health concern, and postharvest practices to improve food safety are essential. While traditional postharvest technologies such as synthetic chemicals have been effective at controlling postharvest decay and maintaining fruit quality during storage, there is an urgent need to develop alternative ‘green technologies’ to maintain product quality through to the consumer. Many new innovative green postharvest technologies are being developed to delay ripening, reduce pathogenic microorganisms, maintain freshness, and improve nutrition. This chapter discusses some new innovative green postharvest technologies such as the application of edible coatings and films, light emitting diode (LED), ultrasound, UVC irradiation, and plasma technology, which have been shown to reduce postharvest losses and improve the nutritional quality of fresh produce.

1. Introduction

Fresh produce including fruits and vegetables are perishable crops. The major problems are quality degradation, microbial spoilage, and postharvest disease infection, resulting in quality and nutritional losses and short storage life. Postharvest handling is a crucial step to maintain the quality of fresh produces. However, applying chemical treatments to reduce microbial growth and inhibit postharvest diseases may cause chemical residue when it is overused, and chemical methods cannot apply to organic produce. Therefore, using postharvest green technology such as ultrasound, light emitting diode (LED), edible coatings, UVC irradiation, and plasma technology to reduce postharvest losses and improve the nutritional quality of fresh horticultural produce may be taken into consideration as an alternative treatment for applying after harvest.

2. Edible coatings and films

2.1 Edible coatings

Edible coatings are a thin layer (˂0.3 mm) of biodegradable materials wrapped or coated around the surface of fruits which act as a barrier for the exchange of moisture and gases between fruits and the surrounding environment which can then delay fruit respiration, ethylene production, and slow down microbial growth [1, 2, 3]. Edible coatings are commonly classified into three main groups. Lipid-based materials (such as fatty acids and waxes), protein-based materials (such as zein, casein, whey protein, soy protein, egg albumin, and gelatin), and polysaccharide-based materials (such as starch, cellulose, chitosan, alginate, and gums) or mixtures of them [4]. These edible coatings are considered ‘green technologies’ which are simple, safe, and eco-friendly and can be applied in liquid form by dipping, spraying, or brushing. The fundamental characteristics of edible coatings are that they must be food-safe, tasteless, odorless, and flexible [5]. Water loss from harvested fruit and vegetables is primarily through transpiration. Edible coatings have been widely reported to reduce weight loss in a range of fruit and vegetables such as sweet cherries, peaches, and plums [6, 7, 8]. Edible coatings primarily reduce quality losses by creating a semipermeable gas barrier around the product to restrict transpiration and regulate gaseous (O₂ and CO₂) exchange between the internal atmosphere of fruits and the external environment [9]. Therefore, edible coatings can enhance shelf life by decreasing weight loss, retarding physicochemical changes, and delaying fruit ripening. Hong et al. [10] reported that the use of 2% chitosan in coating guava fruits reduced weight loss, delayed changes in total soluble solids (TSS) and titratable acidity (TA), and maintained firmness during 12 days of storage at 11°C. Green pepper fruit coated with 2% chitosan exhibited a reduction of weight loss and extend postharvest life for 16 days at 12°C [11]. Similarly, Mandal et al. [12] found that 2% chitosan coatings on mango fruits reduced weight loss, remained green of color peel, and increased the shelf life compared to the untreated fruit at room temperature. Moreover, the application of 1% chitosan also delayed changes in weight loss, TSS, TA, and external color compared to the untreated fruit [13]. In addition, Ruelas-Chacon et al. [14] suggested that guar gum (1.5%) coating had great potential in reducing the respiration rate of tomatoes. Li et al. [15] reported that 1% peach gum polysaccharides coating on the surface of cherry tomatoes decreased weight loss, reduced respiration rate, maintained firmness, and extended the shelf life of cherry tomatoes. Similarly, sweet cherry coated with 5% alginate reduced respiration rate and higher in fruit firmness and TA compared to untreated fruit [16]. Ratra et al. [17] reported that aloe vera gel (50%) reduced weight loss, lower the respiration rate, and increased the shelf life of bananas. Moreover, other research confirmed that sweet cherry, pineapple, and apple benefited from aloe vera coating with significantly lower weight loss and delayed fruit ripening [18, 19, 20]. In addition, ‘Alberta’ peaches coated with methyl cellulose and sodium alginate reduced respiration rate by 62% and 68%, respectively, during storage at 15°C [21]. The mixture of high concentrations of chitosan and low concentrations of glycerol has been reported to decrease the respiration rate and delay the ripening of coated tomatoes [22]. Duong et al. [23] demonstrated that sodium alginate solution mixed with 40 mL L⁻¹ calcium chloride (CaCl₂) significantly reduced the weight loss and respiration rate of the rose apple for 10 days at 13°C, while Mandal et al. [24] reported that 3% carboxymethyl cellulose blended with 2% chitosan and wax extended the shelf life of tomato for 24 days at 20–25°C by delaying the ripening process. These numerous examples clearly demonstrate the application of ‘green’ edible coatings to improve the shelf life of fresh produce and minimize postharvest losses.

2.2 Edible films

Edible films are usually prepared by dissolving in water, alcohol, or a mixture of solvents. A plasticizer is often added to the solution in order to improve flexibility and elasticity of film. Other additives, such as antimicrobial agents, colors, and flavor can be added to the solution to create specific film properties and functionality [25]. Polysaccharide-based edible films such as carrageenan and chitosan can be used as a strong barrier to non-polar aroma compounds, reducing aroma loss and oxidation [26]. Hydrocolloid-based edible films such as alginate and carboxymethylcellulose (CMC) have the potential to prevent moisture losses [27]. Another essential factor to contemplate for selecting edible film material is the ability to serve as an effective carrier for antimicrobials. Edible chitosan film combined with bioactive compounds and essential oils allowed for the reduction of Escherichia coli and Listeria monocytogenes and the enhancement of the overall quality of broccoli [28]. Pullulan-based edible films combined with antibrowning and antimicrobial agents prevented enzymatic browning, delayed tissue softening, decreased weight loss, reduced respiration rates, and inhibited the growth of microorganisms in fresh-cut apples [29]. Although edible coatings and films are not replacing conventional packaging completely, they can be used as alternative packaging. Edible films do not only decrease the postharvest losses but also reduce the environmental pollution in long term.

3. Light emitting diode (LED)

3.1 LEDs induce phytonutrients in fresh produces

Light is an important environmental factor that influences plant growth and development, as it offers both the source of energy for photosynthesis and the signal for a wide range of physiological and biochemical processes. LED technology can produce monochromatic light within a narrow wavelength between 400 nm and 700 nm, where LEDs of different wavelengths (such as red, blue, green, and white light) can trigger different responses in plants. The blue and red LED lights are the most effective wavelengths for plant photosynthesis, while yellow and green light have a negligible effect because the absorption spectra of the photosynthetic pigments mainly target blue (400–500 nm) and red (600–700 nm) light spectra [30]. While LEDs are used widely in protected cropping, LED lights are becoming widely studied for their postharvest applications to extend the shelf life and maintain the postharvest quality of fresh produce due to their wavelength specificity, long lifespan, low thermal energy, and non-toxicity [31]. The quality of light has been shown to affect the accumulation of phytonutrients and enhance the levels of phytonutrients in plants [32]. Ambient light supplemented with blue, red, green, or blue: red LEDs enhanced the total phenolics, flavonoids, anthocyanins, lycopene, α-tocopherol, and other compounds in several fruits and vegetables (Table 1).

LED light	Light conditions	Fresh produces	Phytonutrients	References
	470 nm, 40 W m^−2	Strawberry	Total phenolics and anthocyanins	[33, 34]
	525 nm, 20 W m^−2	Broccoli	Total phenolics	[35]
Blue	436 nm, 1.52 W	‘Dongdori’ cabbage	Total phenolics, chlorophyll, and vitamin C	[36]
	400–500 nm, 80 μmol m^−2 s ^−1	Purple pepper	Anthocyanins	[37]
	625 nm, 12 W	Table grapes	Total phenolics and flavonoids	[38]
	663 nm, 1 W	Cherry tomato	Lycopene and β-carotene	[39]
Red	638–nm and 665–nm, 210 μmol m^−2 s^−1	Basil and parsley microgreens	Total phenolics, α-tocopherol, and vitamin C	[40]
Green	510 nm, 30 μmol m^−2 s^−1	Baby leaf lettuce	α-Carotene and anthocyanins	[41]
	52 μmol m^−2 s^−1	Chinese cabbage	Total polyphenols and total flavonoids	[42]
Blue:red	Blue (30%) and red (70%)	Eggplant	Phenolic acid, chlorogenic acid, and gallic acid	[43]
	2016 kJ m^−2, 16 h	Carrot sprouts	Carotenoids	[44]

Table 1.

Effect of LEDs in inducing the levels of phytonutrients on fresh produces.

LEDs’ role in the induction of bioactive compound production in plants seems to be associated with phenylalanine ammonia-lyase (PAL) enzyme, which is engaged in the initial step of the phenylpropanoid pathway [45]. It has previously been reported that LED increased accumulation of primary metabolites in plants and induced the suppression of the translocation of photosynthetic products. LEDs also affect the signal transduction pathways by inducing the production of secondary metabolites in plants [46]. LED lights have a significant effect on the accumulation of phytonutrients and therefore could be used as an alternative technique for enhancing the quantity and quality of the phytonutrient profiles linked to nutrition and human health. However, the use of single- or combination-spectral light ratios may vary effect depending on the plant species or cultivars [47]. Therefore, more investigation is required to establish the spectrum qualities that make the best choice for enhancing the phytonutrient properties of fresh produce.

3.2 LEDs induce disease resistance in fresh produces

Nowadays, LEDs are becoming increasingly popular as a practical tool for protecting fruits from pathogen attacks. The specific wavelengths of light, especially red and blue LEDs, can induce disease resistance in plants against a wide range of microorganisms (Table 2). The mechanism of LED inactivation against pathogens is direct damage to DNA and cell membrane caused by reactive oxygen species (ROS) produced by LED light [54]. Blue light enhanced the production of phytoalexin scoparone thus, preventing the postharvest decay caused by Penicillium digitatum and P. italicum in citrus fruits [55]. Moreover, it was also reported that blue light stimulated the formation of phospholipase A2 and octanal, which resulted in a reduction of P. digitatum and P. italicum activities [50]. Red light inhibited disease development by increasing the expression of defense-related genes and promoting the production of phytoalexin stilbenenic [56]. LED lights affect the preservation of fresh produce after harvest by a dual effect; 1) the inhibition of microbial growth and 2) stimulation of plant defense responses. Therefore, further investigation is required to truly understand the mechanism, which may potentially lead to the development of LEDs that are effective tools for reducing postharvest disease.

LED light	Light conditions	Fresh produces	Effect on disease	References
	200 μmol m^−2 s^−1	Lettuce	Induced resistance against gray mold by Botrytis cinerea	[48]
Blue	50–150 μmol m^−2 s^−1	Tomato	Induced resistance against gray mold by B. cinerea	[49]
	40 μmol m^−2 s^−1	Tangerine	Induced resistance against P. digitatum	[50]
	8 μmol m^−2 s^−1	Mandarin	Induced resistance against P. italicum	[51]
Red	287 μ W cm ^−2	Bell pepper	Induced resistance against Phytophthora capsici	[52]
	80 μmol m^−2 s^−1	Grapevine leaves	Induced resistance against gray mold by B. cinerea	[53]

Table 2.

Effect of LEDs in the inducing disease resistance in fresh produces.

4. Ultrasound

Treatment of fruit and vegetables with ultrasound is a relatively new method of maintaining fruit and vegetable quality after harvest. Ultrasound is a type of vibrational energy in the frequency above 20 kHz, which is higher than the range of human hearing. Based on the frequencies, ultrasound waves are divided into three categories, including diagnostic ultrasound (1–500 MHz), high-frequency ultrasound (100 kHz–1 MHz), and low-frequency ultrasound (20–100 kHz). The applications of ultrasound are classified into low and high energy depending on the sound power (W), sound intensity (W m⁻²), or sound energy density (W s m⁻³). High-energy ultrasound with low frequencies (20–100 kHz) refers to as ‘power ultrasound,’ which is widely used in the food industry to preserve food quality and provide food safety [57]. The principle of ultrasound on microbial inactivation is to produce a cavitation phenomenon through the liquid medium. During the sonication process, longitudinal ultrasound waves pass through liquid media and produce alternating regions of compression and rarefaction to promote the cavitation phenomenon. A certain amount of gas can be trapped in cavities and form cavitation bubbles in the regions of different pressure. During rarefaction, the negative pressure increases the bubble volume, while the positive pressure decreases the bubble volume as the sound wave changes to compression. During treatment, very localized high temperatures (5500 K) and pressures (up to 50–100 MPa) are created by shock waves, which also accelerate bubble collapse. When the bubble collapses, the breakdown of water molecules inside the bubble explosion produces free radicals such as hydrogen atoms [H⁺] and hydroxyl radicals [OH⁻]. Hydrogen peroxide [H₂O₂] is formed when OH radicals combine with water [H₂O], which is a cause of disruption of the cell wall, thus inhibiting the growth of microorganisms [58].

Microbial spoilage is a major cause of postharvest waste and is caused by fungi, bacteria, yeast, and molds. Fresh produces are normally protected from microbial attack by using synthetic chemicals, but the application of green technologies such as ultrasound also reduce postharvest losses. Cao et al. [59] reported that ultrasound treatment at 40 kHz and 250 W for 10 min could be applied to reduce mesophilic aerobic mold and yeast by 1.49 log CFU g⁻¹ and 1.73 log CFU g⁻¹, respectively, and maintain the quality of strawberry at 5°C for 8 days. Birmpa et al. [60] reported that ultrasound treatment at 37 kHz and 30 W for 45 min was effective in reducing the growth of E. coli, Listeria innocua, Salmonella enteritidis, and Staphylococcus aureus inoculated on lettuce and strawberries. Similarly, Alexandre et al. [61] showed that the number of L. innocua inoculated on red bell pepper was reduced by 1.9 log CFU g⁻¹ after being treated with ultrasound treatment at 35 kHz and 350 W for 2 min. Gani et al. [62] observed that the use of ultrasound treatment of strawberries at 33 kHz and 60 W for 40 min contributed to a reduction in the number of bacterial from 5.91 to 3.91 log CFU g⁻¹ and yeast and mold from 4.80 to 3.58 log CFU g⁻¹. Supapvanich and Kijka [63] demonstrated that the ultrasound treatment at 40 kHz and 150 w for 10 min maintained visual appearance, inhibited decay incidence, and reduced weight loss of ‘Kim Ju’ guava fruits during storage at 28 ± 1°C for 6 days. Furthermore, Guerrero et al. [64] confirmed that ultrasound treatment damages the cell wall and membrane integrity of the microbial pathogen. Thus, the ultrasound treatment reduces postharvest microbial growth due to cellular disruption by shear force or increasing temperature and pressure during bubble collapse, which creates hydroxyl radicals and leads to severe cell wall damage [65]. The produced highly reactive radicals can destroy the cell wall and membrane of microorganisms [66], which helps in the prevention of postharvest disease in fresh produces.

While ultrasound treatment has been shown to reduce the levels of microbes in fruits and vegetables, its application in combination with other green technology treatments, such as sanitizers, organic acids, essential oils, and other antimicrobials, has been studied to enhance the efficiency of this technique. For example, Chen and Zhu [67] reported that the combination of ultrasound at 40 kHz and 100 W for 10 min with chlorine dioxide at 40 mg L⁻¹ reduced initial microflora and maintained the quality of Japanese plum. Similarly, the population of Salmonella and E. coli O157: H7 contaminated on the surface of apples was reduced by 2 and 1.5 log CFU g⁻¹ in combination with ultrasound at 170 kHz and chlorine dioxide at 40 mg L⁻¹ for 10 min [68]. Sagong et al. [69] observed that the effect of 2% lactic acid, 2% citric acid, and 2% malic acid combined with ultrasound treatment at 40 kHz for 5 min was effective to reduce E. coli, S. typhimurium, and L. monocytogenes on organic fresh lettuce. Yang et al. [70] also reported that the combination of ultrasound at 40 kHz for 10 min with salicylic acid mM could reduce blue mold caused by Penicillium expansum in peach fruit. Millan-Sango et al. [71] demonstrated that the combined treatment of ultrasound at 26 kHz and 200 W for 5 min with thyme essential oil at 0.018% reduced Salmonella enterica in lettuce. These examples demonstrate the effectiveness of ultrasonic combination treatments to reduce food safety and fruit pathogens; however, more work is required to optimize these treatments to kill pathogens and have minimal effects on product quality.

5. Ultraviolet (UV) radiation

UV radiation is widely used as a microbial sterilizing treatment in a range of diverse applications such as the treatment of wastewater, process water, surface disinfection, and air disinfection in food processing, packing, pharmaceuticals, and hospitals [72, 73]. In the postharvest treatment of fresh produce, UV radiation has been used for more than four decades. The main uses of postharvest UV treatments in postharvest include the following: (1) its use as defect sorting, that is, separating defective fruit (discolored and wounds) from perfect fruit, (2) control of spoilage and pathogenic microorganisms (i.e., Alternaria sp., B. cinerea, Colletotrichum sp., Lasiodiplodia theobromea, Monilinia sp., Fusarium sp., Rhizopus sp., and Penicillium spp.), (3) enhancement of bioactive compounds, (4) delayed fruit ripening and senescence, and (5) control of human pathogens (i.e., E. coli, Salmonella, and Listeria) that can contaminate fresh produce [72, 73]. However, this review will focus on the application of UV-C in the control of plant pathogens that cause postharvest diseases and improving the quality of fresh produce. UV radiation is commonly considered non-ionizing radiation where the wavelength of UV radiation is 100–400 nm, and its light region is between X-ray and visible light spectrum. The electromagnetic spectrum of UV is commonly divided into three regions: UV-C is a short wavelength of 100–280 nm, UV-B is a medium wavelength of 280–315 nm, and UV-A is a long wavelength of 315–400 nm [73]. Among these different UV spectra, UV-C shows the highest antimicrobial effect [72]. The most efficient wavelength for damaging genetic materials (DNA) of microbial and all biological tissues and eliciting antimicrobial compounds is 254 nm [72, 74]. Microbial death caused by UV-C is not only by genetic material damage but also by the overproduction of ROS, which can oxidize membrane lipids and inactivate the activity of cellular enzymes. It has been shown that gram-negative bacteria are more sensitive than gram-positive bacteria, followed by yeasts and eukaryotic organisms [73].

5.1 UV-C treatment in the control of postharvest disease

Postharvest disease is one of the major economic losses of fresh produce where the most postharvest diseases are caused by fungi, yeasts, and bacteria. Synthetic fungicides are currently used to prevent disease in fresh fruit and vegetables, but consumers and regulators are seeking safe and effective alternative treatments to manage postharvest decay. Treatments such as UV-C are ideal alternatives to synthetic chemicals as it is a physical treatment and does not leave chemical residues [72]. The UV-C desired dose required controlling postharvest diseases, and improving produce quality is dependent on a range of factors such as the type of produce, maturity stage, cultivar, and harvest season, but generally ranges between 0.2 kJ m⁻² and 20 kJ m⁻² [72, 73].

The antimicrobial effect of UV-C treatment on the control of postharvest diseases is thought to occur through two different mechanisms: (1) a direct germicidal effect on biomolecules of plant pathogens such as nucleic acid, membrane, and proteins [75] and (2) via an indirect effect on plant pathogens through the induction of the plant defense mechanisms following UV-C treatment such as plant defense enzymes, plant pathogenesis-related proteins (PRs), and secondary metabolite accumulation (i.e., antimicrobial agents—phenolic compounds and phytoalexins) [73, 76]. The limitation of a direct effect of UV-C treatment against pathogens is the low penetration of this radiation treatment into plant tissue, where only pathogens present on the surface can be killed due to the poor penetration ability of UV-C into the tissue (only 50–300 nm) [72]. Thus, to get complete coverage fresh produce must be rotated during treatment to ensure that all surfaces are exposed to UV-C light.

In general, plant defense responses can be induced by biotic and abiotic elicitors such as pathogens, antagonistic microorganisms, chemical treatments, and physical treatments like UV-C irradiation. Several studies have shown that UV-C treatment can elicit a range of responses such as the accumulation of antimicrobial compounds, the increase in the activities of various enzymes associated with plant defense, and plant pathogenesis-related proteins (PRs) such as phenylalanine ammonia-lyase (PAL), chitinase (CHI), β-1,3 glucanase (GLU), and peroxidase (POD). PAL is the key enzyme that plays a role in the biosynthesis of phenolics and other secondary metabolites phytoalexins (phenol), and even salicylic acid, which is a crucial signal molecule involved in plant protection from pathogen attack [75, 77]. UV-C radiation has been shown to induce plant defense mechanisms in a range of different fresh produce such as strawberry [74], pear [75], mangosteen [76], mango [78], peach [79], tomato [80, 81], and banana [82], thereby extending the shelf life of these fruits.

5.2 UV-C treatment in postharvest nutritional values

UV-C has been shown to affect a range of postharvest quality attributes including changes in plant pigments, antioxidants, nutrition, firmness, flavor, and aroma of fresh produce. These physiological changes may be desirable or undesirable depending on the type of produce as these changes directly affect eating quality. UV-C treatment has been shown to enhance antioxidant systems for both antioxidant compounds and oxidative enzyme activities in various fresh produce pear [75], mangosteen [76], mangoes [78], broccoli [83, 84], leaf vegetables [85], garlic [86], pepino fruit [87], and tomato [88, 89]. While many studies have shown that UV-C radiation treatment can induce many bioactive compounds, but some studies have shown that UV-C treatment did not enhance polyphenols, β-carotene, ascorbic acid, chlorophyll contents in persimmon and cucumber [90], and total phenolic content, anthocyanin compounds, and antioxidant activity in grape [81].

5.3 Combined UV-C treatment with other postharvest technologies

UV-C treatment can combine with other postharvest treatments such as chemical, physical, and biological treatments to improve postharvest quality. For example, Sripong et al. [78] demonstrated that UV-C irradiation (6.16 kJ m⁻²) combined with hot water treatment at 55°C for 5 min significantly reduced the incidence and severity of postharvest anthracnose disease development in mango fruit compared with UV-C or hot water treatment alone. The reduction of disease was related to an increase in plant defense-related enzyme activities and their related gene expressions. In addition, the combined treatments could retard fruit ripening by maintaining firmness, slowing color change, retarding the increase of total soluble solids, and the decrease of titratable acidity (TA). This result could be explained that the treatments may slow down the activity of the enzymes associated with plant cell wall degradation and the hydrolysis of starch to sugar. The acidity of fruit has been known that it is used as a respiratory substrate. Thus, low respiration rate is a cause of slowing down the decrease in TA.

5.4 The advantage and disadvantages of UV-C treatment

As a physical treatment to improve the storage life of fruit produce, UV-C treatment has numerous practical advantages that include the following: (1) fast and simple handling, (2) low cost and investment, (3) low maintenance, (4) ability to operate at low temperatures and not requiring additional water, (5) less space for treatment, (6) it is non-ionizing treatment, (7) it is non-thermal technology which does not induce heat in the tissue of fresh produce, (8) fewer regulatory restrictions as compared to other irradiation methods such as gamma irradiation, and (9) approved by food control agencies [72, 78]. However, the disadvantages of UV-C radiation include the following: (1) low penetration power, and the potential penetration into plant tissue is only 50–300 nm. Thus, only pathogens on the plant surface are inactivated after exposure to this radiation, (2) UV-C rays are dangerous a workplace health and safety (WHS) issue and can damage the eyes and skin [72]. Thus, during UV-C treatment application even on the laboratory scale and a commercial scale, the treatment must be conducted with complete protection to all users, (3) generation of ozone gas during the operation with UV radiation, particularly at a wavelength below 260 nm, will produce ozone gas. Ozone is hazardous to the human health and can cause the air pollution in the working atmosphere, and thus, air ventilation or treatment may be necessary. The U.S. Environmental Protection Agency (EPA) states that the national ambient air quality standard for ozone is 70 ppb for an 8-hour average [82]. However, activated carbon filters are being used for ozone removal in the atmosphere [83, 84]. In summary, UV-C technology is a potential physical treatment that has many benefits for the storage of fresh fruit and vegetables. UV-C treatment has been shown to induce defense systems within the fruit through the signaling molecules and antimicrobial compounds to protect against plant pathogens, improve the nutritional quality of some fresh produce, and delay senescence resulting in extending the shelf life of fresh fruit and vegetables.

6. Plasma technology

6.1 Types of plasma and plasma generation

Plasma technology is a novel approach to preserving the quality and inactivating spoilage microorganisms and pathogens of food processes and maintaining the postharvest quality of fresh produce. It is an accepted technology for industry because of its relatively low input cost [85]. Plasma is commonly known as the fourth state of matter, after gases, liquids, and solids. Plasma is quite like a gas than a solid or liquid, but its property differs from solid, liquid, and gas [86]. Two major types of plasma are differentiated based on thermodynamics: thermal plasma and non-thermal plasma (referred to cold plasma, temperature < 60°C) [86, 87, 88]. Thus, cold plasma (CP) is selected to apply to various foods including fruit and vegetables to avoid the loss of nutrition and sensory contributions. CP can be generated by many techniques including corona discharge (CD), dielectric barrier discharge (DBD), gliding arc discharge (GAD), microplasma, radio frequency (RF) discharge, plasma spray, plasma needle, and atmospheric pressure plasma jet [86, 88]. In all plasma techniques, the feed gas is required to energize into plasma [88]. CP is commonly generated by energizing matter with an electric current (energy), and the solid state of matter changes from liquid to gas and finally to a plasma state, respectively. When the initial gas is treated with sufficiently high energy, the structure of molecules and intra-atomics is broken resulting in the formation of free electrons, neutrals, and other reactive ion species. The initial feed-in gas for plasma generation can be either a single gas or a mixture of various gases, i.e., carbon dioxide, oxygen, nitrogen, argon, neon, helium, or even air [86, 87]. In addition, moisture can be mixed with the feed-in gases [88]. The treatment plasma state consists of a mix of excited atoms and molecules (ionization), positive and negative charged particles, UV photons, ozone, ROS (such as superoxide [O₂⁻], hydroxyl [OH˙·], hydroperoxyl [HO₂]), hydrogen peroxide [H₂O₂]), and reactive nitrogen species (RNS such as nitric oxide [NO], nitrogen dioxide [NO₂], dinitrogen pentoxide [N₂O₅], nitrate anions [NO₃⁻], and nitrite anions [NO₂⁻]) [86, 87, 88].

Sufficient concentrations of the plasma compositions in the environment (atmosphere or liquid plasma) have been shown to inactivate microbes such as bacteria, fungi, spores, and viruses without hazardous chemical residues [86, 89]. CP can be subdivided into non-thermal atmospheric plasma (NTAP) and plasma-activated water (PAW). PAW is conducted by applying plasma discharge in water (plasma generated directly in the water) or on the water surface (plasma generated over the water surface) [90, 91]. An increase in many chemically reactive molecules (reactive oxygen and nitrogen species, RONS) created in PAW is correlated with high oxidation-reduction potential (ORP), electrical conductivity (EC), and low acidity pH levels [92] that are considered to have high antimicrobial effects. However, the use of some oxidizing chemical solutions instead of water for plasma generation is called the plasma-activated solution (PAS) and can be used to increase the antimicrobial mechanism of the treatment, as indicated by the increases in ORP and EC values [92]. ORP, EC, and low pH values provide synergistic inactivation effects along with RONS. In addition to anti-microbial effects, CP can significantly slow the loss of fresh produce quality deterioration via the slowdown ripening and senescence processes, thereby maintaining the nutrition and organoleptically appearance [91].

6.2 Mode of action of cold plasma

There are large numbers of studies examining the mechanisms of CP treatment against microbes. It is generally thought that the mode of action of CP involves the interaction of the active chemical species, radicals, and reactive molecules generated by CP with microbial cell membranes and cellular functions through lipid peroxidation, protein denaturation, and genetically material degradation leading to cell damage, pore formation, cell lysis, and cytoplasm shrinkage [85]. However, the long lifespan of RONS such as hydrogen peroxide, nitrate, nitrite, and ozone may help maintaining the inactivation effect on microbial for a longer period than short-lifespan species such as hydroxyl radicals, superoxide, singlet oxygen, nitric oxide, peroxynitrite, and peroxynitric [92].

6.3 Cold plasma for food-borne pathogens

There are many studies on application of cold plasma treatment for inhibiting food-borne pathogens contamination, postharvest disease infection, and maintaining the quality of fruit and vegetables. For example, the inactivation effects of cold plasma on food-borne pathogens have been reported in a large number of produce varieties: Salmonella typhimurium in radish sprouts [93], Salmonella stanley and E. coli O157: H7 in apples [94], E. coli O157:H7 Salmonella sp., L. monocytogenes in apples, cantaloupe, and lettuce [95], etc.

As a case study, the antimicrobial efficiency of PAS treatment on cell structure of the food-borne pathogen and the contaminated fresh lettuce was investigated and showed that a 3% (v/v) of H₂O₂ solution treated with DBD plasma and incubated with Staphylococcus aureus for 10–30 min resulted in a decrease in the PAS-treated S. aureus population, which was correlated with the increase in PAS incubation time. Although the cell wall of gram-positive S. aureus is thick, it is composed of a peptidoglycan (PG) layer, which is less sensitive to ROS. It is thought that the high ROS formation by PAS attacks the PG layer, membrane, and DNA leading to cell death. Higher leakage of intracellular components DNA, proteins, and K⁺ of PAS-treated S. aureus were also found in longer incubation time than in short incubation time. In vitro tests where soaking lettuce samples with PAS for 10 min have been shown to reduce the total bacteria counts by 1.25 log CFUg⁻¹, and this treatment also retained the nutrient quality of lettuce, as indicated by the stability of green color and vitamin C and chlorophyll contents when compared with the control (deionized water) [96].

6.4 Cold plasma for postharvest quality and disease control

Postharvest decays caused by microbial contamination and infection are the major factors of postharvest deterioration. The effects of plasma treatments against various postharvest diseases in a range of different fresh produce have been reported. For example, the application of CP treatment on postharvest fungal pathogens and spoilage fungi has been reviewed in Aspergillus niger in date palm [97], P. italicum in Satsuma mandarins [98], P. digitatum in lemon [99], Alternaria alternata, A. niger, and P. italicum in wash water from cherries, grapes, and strawberries [100].

PAW treatment for 45 min has been shown to inhibit postharvest decay of kumquat fruit caused by Penicillium italicum and maintained the fruit firmness, while color, vitamin C, total flavonoid, and carotenoids were not affected by PAW treatment [101]. The results of decontamination of strawberries inside closed packages with different gas mixtures (O₂, CO₂, and N₂), and then treated with DBD plasma at the ambient temperature had a similar antimicrobial effect to decontaminate total aerobic mesophiles and yeasts and molds by 3.0 log reductions. Strawberry fruit treated with plasma at high oxygen gas mixtures (65% O₂ + 16% N₂ + 19% CO₂) had lower fruit respiration and higher firmness and brightness than plasma treatment at a high nitrogen gas mixture (90% N₂ + 10% O₂) [102]. The quality of the mushroom (Agaricus bisporus) after soaking with PAW for 1, 10, and 15 min showed that PAW treatment reduced bacterial and fungal contamination by 1.5 and 0.5 log and also delayed the mushroom softening. In addition, mushroom color, pH, and antioxidant properties were not affected [103].

The effects of plasma treatment on either postharvest disease inhibition or quality maintenance in tropical fruit have been reported. For example, the plasma-activated solution (PAS) obtained by treating with a dielectric barrier discharge (DBD) in the phosphate buffer showed a decrease in spore germination and spore viability of Colletotrichum asianum, a causal agent of mango anthracnose disease. Indeed, the incidence of Colletotrichum in PAS-treated mango fruit was 48% lower than in non-treated fruit. SEM microstructure of the treated Colletotrichum spores showed that the subcellular structure was damaged by plasma treatment [104].

Browning of fresh and minimally processed fresh produce is an undesirable characteristic that is involved with two major browning enzymes: polyphenol oxidase (PPO) and peroxidase (POD). In postharvest trials with treated banana fruit, the PPO and POD activities of the atmospheric plasma-treated banana slices were 70% and 100%, respectively, lower than untreated fruit. In addition, the levels of phytonutrient compounds (total phenol and flavonoid), antioxidant activity, and vitamin B6 also increased following treatment. However, the plasma treatment produced porous structures in the fruit leading to a decrease in hardness [105]. A similar observation resulted in other studies with banana slices has shown that CP treatment inactivated the enzymatic browning activities (PPO and POD) resulting in low quinones and less browning. However, plasma treatment also induced ROS levels in cells and morphological surface changes (rougher, fissures, and cracks) with the increase in treatment time [106].

The shelf life of fruit and vegetables is usually limited by plant pathogens and senescence as indicated by their respiration rate. Cold storage and modified atmosphere packaging (MAP) are commercial preservation technologies, which are commercially used to prolong the shelf life of fresh produce [107]. The combined effect of cold storage, MAP, and plasma treatments on the safety and quality of cherry tomatoes has been studied. Cherry tomatoes treated with PAW and packed in the polypropylene (with perforated-oriented polypropylene film to generate the equilibrium MAP condition) and then stored were found to have reduced the microbial load on the surface of the tomato, whereas MAP and low storage temperature could prevent water loss and maintain the total soluble solids content resulting in shelf life extension [108]. However, there is less information about the effect of cold plasma on the activation of the natural defense mechanisms in fresh produce, and more work is required in this area.

6.5 Advantages and disadvantages of plasma technology

Cold plasma is heat-free technology that is suitable for fresh produce as it prevents postharvest losses by inactivating plant pathogens, reducing microbial contamination, slowing the loss of freshness and firmness, and delaying nutritional losses: vitamins, colors (i.e., chlorophyll, carotenoids, and anthocyanin), various phytochemical compounds, antioxidant properties, without no harmful synthetic chemical residues [86]. However, the efficiency of cold plasma technology for decontamination and quality maintenance is dependent on the variety of gas, gas flow rate, and process time of treatment. Undesirable conditions related to these factors may provide negative effects such as loss of color, nutrition, and bioactive compounds. For example, the decline in brightness (L^* value) of strawberry fruit treated with atmospheric cold plasma under a high nitrogen environment is probably due to superficial bleaching [102]. The total color difference (ΔE^∗) value of the carrot slices treated with atmospheric pressure cold plasma treatment is unacceptable and may be caused by high oxidation of the surface carotene and the loss of moisture on the surface. A major disadvantage of the use of CP is the high investment in a state-of-the-art cold plasma system [86].

7. Conclusion

Postharvest losses represent economic losses in fresh food supply chains but can be drastically reduced and prevented by using appropriate postharvest technologies. Reducing postharvest losses have the potential to enhance food availability and minimize waste. Green postharvest technologies such as those presented above including light emitting diodes, edible coatings, and UV can be applied to fresh produce to enhance nutritional values and also maintain postharvest quality. In addition, the applications of ultrasound, UV-C, and plasma technology could reduce postharvest diseases, eliminate microbial contamination, and improve food safety, resulting in reducing food loss and food waste. However, there are still many technical and commercial challenges that need to be overcome to maintain the quality and extend the shelf life of fresh produce for modern trade. Each green technology has desirable limitations, and thus, using the hurdle approach to improve its effects on the optimization of the combined treatments may be a focus of future research for industrial application. For example, the application of ultrasound combined with UVC may have synergistic effects on microbial reduction. The combined technologies including edible coatings/films together with UVC, LED with UVC-LED, or hot water treatments with plant extracts or essential oils can be improved bioactive compounds and antioxidant activity. More research is needed to provide efficiency and cost-effectiveness for industry.

Acknowledgments

This is in part a contribution from the Horticulture Innovation ‘Citrus Postharvest Program’ (CT19003).

Conflict of interest

The authors declare no conflict of interest.